The present invention is directed to a functionalized metal substrate and method for detection of anions of concern in environmental samples.
Perchlorate (ClO4−) has been detected recently in groundwater, surface water, and soils and, more ominously, in plants, food products and human breast milk in many areas of the United States and the world. Most perchlorate is manufactured for use as a primary ingredient of solid rocket propellant and explosives. However, perchlorate is also used in pyrotechnic devices, such as fireworks, highway flares, gun powder, air bags, and in a wide variety of industrial applications such as tanning and leather finishing, rubber manufacturing, and paint and enamel production. Naturally-occurring perchlorate is also known to exist. As a result, the widespread use and the presence of both natural and anthropogenic perchlorate have caused widespread contamination in groundwater and drinking water supplies. For example, the entire Lake Mead and the lower Colorado River are contaminated with perchlorate, affecting millions of people and agricultural lands. Because of its potential health affect on thyroid function and hormone production by interfering with iodide uptake, the widespread occurrence of perchlorate in the environment has resulted in intense public debate and far-reaching ramifications, ranging from public health issues to liabilities that could be imposed by environmental cleanup needs.
Perchlorate is also exceedingly mobile in aqueous systems and can persist for many decades under typical groundwater and surface water conditions. Many states have already set regulatory or advisory levels of perchlorate in drinking water, ranging from 1 to 18 μg/L (e.g., 1 μg/L in Maryland, Massachusetts, and New Mexico, and 6 μg/L in California). Therefore, methods for a rapid and sensitive assay of this contaminant are urgently needed to allow continuous monitoring and detection of this contaminant in groundwater and drinking water. At present, ion chromatography (IC) with conductivity detection is the recommended method by EPA for quantitative analysis of perchlorate. Its detection of perchlorate is based on the retention-time when perchlorate is eluted off an IC column. Therefore, this method is not only non-selective but also requires a lengthy analytical time. IC has a detection limit of ˜1 ppb for perchlorate in a relatively pure water analysis but exhibits problems for analysis in some environmental samples due to interferences from other dissolved ions or species in water. For example, in fertilizer analyses, the concentration of perchlorate is typically orders of magnitude less than that of other oxyanions (e.g., nitrate, sulfate, and phosphate) that are usually present in the fertilizer extracts. At high TDS (total dissolved solids) concentration, the IC peaks broaden due to column overloading to the extent of obscuring the less-prominent perchlorate peak. Even when the perchlorate peak is not completely obscured, tailing associated with column overload may add errors in peak area integration. In addition, the detector overload due to high TDS concentration may severely affect baseline response. Therefore, to successfully perform an analysis for perchlorate at a high TDS concentration, a tedious pretreatment including dilution, cleanup procedures is required. Such a pretreatment significantly increases the total time and labor required for analysis. Other problems that IC exhibits for analysis of perchlorate in complex matrices include retention time migration with column deterioration, detector fouling, and long data acquisition time. IC coupled with mass spectrometry (IC-MS) has also be used with a better sensitivity (˜0.01 ug/L) but the analytical cost is enormous, and the analysis is also subjected to interferences by the presence of other ions and impurities in environmental samples
For long-term monitoring, it is desirable to detect perchlorate ions in situ to minimize sample volume, handling time, and costs. Based on recent studies of the inventors, an effective method for rapid, sensitive, and in situ detection of ClO4−, as well as other anionic contaminants including radioactive technetium and uranium, can include surface-enhanced Raman scattering (SERS) analysis. Portable Raman spectrometer systems coupled with fiber-optic probes are now commercially available and are relatively inexpensive, robust, and require only minimal sample preparation and handling. The characteristic vibration frequency of the symmetric stretch for ClO4− at ˜950 cm−1 (dehydrated) and ˜934 cm−1 (in aqueous solution) makes the technique especially selective. Using unfunctionalized silver nanoparticles as substrates, we recently reported a detection limit of 100 μg/L by SERS. An even lower detection limit (˜10 μg/L) was achieved by first concentrating ClO4− onto a bifunctional anion-exchange resin followed by the normal Raman spectroscopic detection.
Accordingly, there exists a need for sensitive and stable SERS substrate materials in order to enhance the detection limit of perchlorate and other anionic chemical species in the environment.